Historically, fan cowl losses have been primarily attributed to human error. This accident involved similar attributes to those previous events. Specifically, the left fan cowl had been opened for maintenance immediately before the occurrence; the fan cowl loss occurred during the take-off roll, when differential air pressures would affect an improperly locked cowl; and there was no damage to the adjacent fan cowl latching mechanism that would be expected if the cowl had failed when the latches were engaged with the keepers and properly locked. Each of these attributes can be used to infer that the cowl had not been properly locked after the ramp maintenance activity on the left engine thrust reverser. Conversely, the forward and the centre latch mechanisms of the failed fan cowl were recovered in the clasped position, indicating a possibility that the cowl latches either had been clasped during the maintenance activity or engaged and properly locked after maintenance and before the failure. Because of the ambiguity of the available information, it is difficult to accurately determine the cause of the failure. However, by looking beyond this particular event, the investigation was able to identify related safety deficiencies involving human factors and mechanical components of the system that, if left uncorrected, could lead to an increased risk of further loss.Analysis Historically, fan cowl losses have been primarily attributed to human error. This accident involved similar attributes to those previous events. Specifically, the left fan cowl had been opened for maintenance immediately before the occurrence; the fan cowl loss occurred during the take-off roll, when differential air pressures would affect an improperly locked cowl; and there was no damage to the adjacent fan cowl latching mechanism that would be expected if the cowl had failed when the latches were engaged with the keepers and properly locked. Each of these attributes can be used to infer that the cowl had not been properly locked after the ramp maintenance activity on the left engine thrust reverser. Conversely, the forward and the centre latch mechanisms of the failed fan cowl were recovered in the clasped position, indicating a possibility that the cowl latches either had been clasped during the maintenance activity or engaged and properly locked after maintenance and before the failure. Because of the ambiguity of the available information, it is difficult to accurately determine the cause of the failure. However, by looking beyond this particular event, the investigation was able to identify related safety deficiencies involving human factors and mechanical components of the system that, if left uncorrected, could lead to an increased risk of further loss. Human Factors Issues Because of the fan cowl's design and method of closure, a fan cowl that has been lowered but not latched looks virtually identical to a fan cowl that has been lowered and properly latched. Latch handles are very difficult to see because of their location on the underside of very large, low-slung engines; this difficulty increases as you move closer to the engine. The company's attempts to make the latch handles more conspicuous by painting them bright red were ineffective because the adjacent cowl area was also painted bright red. This paint scheme effectively camouflaged the fan cowl latching mechanisms. The inappropriate use of tools to operate these latches can damage the latch's internal spring mechanism. Additionally, increased leverage on the handle, as a result of tool use, may make the bending or breaking off of a latch's trigger/safety pins more probable, thereby negating their safety function. This problem could be corrected by clarifying the maintenance manuals, specifically warnings about the use of tools. In theory, the fan cowl latches should have been inspected by the flight crew before departure. In practice, however, the flight crew walk-around inspection was conducted before the maintenance items were complete. At that time, numerous doors and latches were open to allow refuelling, resupply of food and beverage, loading of baggage, loading of passengers, and maintenance activity on the left engine. The effectiveness of the flight crew's pre-flight inspection to confirm the completeness of a maintenance activity was lost. Difficulties in visually detecting unsecured latches, the lack of a final flight crew pre-flight inspection as a defence to ensure that fan cowls are properly secured, and the industry-wide practice of using tools to operate the latches all increase the risk that a fan cowl failure will occur because of human error. Mechanical Deficiencies Of the fan cowls examined during this investigation, approximately 80% were below the aircraft manufacturer's specified minimum latch tension. Low tension increases the likelihood that a latch will become unlatched on its own. For that reason, the present requirements regarding tensioning do not appear to be appropriate. Deficiencies related to the orientation of latch fitting sub-assembly bolts with the threads inward, the interference between latch fitting sub-assembly fasteners and trigger/safety pins, the adjustment star retention spring failures, the insufficient latch tensioning, and the incomplete trigger/safety engagement will increase the risk of fan cowl loss due to mechanical malfunction. More than one cowl latch was not secured or failed to lock properly, thus allowing the fan cowl to open during take-off.Findings as to Causes and Contributing Factors More than one cowl latch was not secured or failed to lock properly, thus allowing the fan cowl to open during take-off. There is no cockpit indication to notify the crew whether fan cowls are closed and secured. Interaction between the trigger/safety pins and the fitting sub-assembly fasteners can damage the ends of the pins and thereby increase the risk of component failure. Orientation of latch fitting sub-assembly bolts with the threads inward and use of longer-than-required bolts exacerbates any interference between the assembly bolts and the trigger/safety pins. Failure of adjustment star retention springs increases the risk that the engine fan cowl latch tension will move out of adjustment. By design, the height of the latches and their position at the bottom centreline of the fan cowls makes it difficult to visually detect an unlatched latch. The present maintenance requirements regarding latch tensioning do not appear to be adequate to ensure that latch tension remains in accordance with the manufacturer's specified minima. The effectiveness of the flight crew's pre-flight inspection was lost when the inspection was done before completion of the maintenance activity.Findings as to Risk There is no cockpit indication to notify the crew whether fan cowls are closed and secured. Interaction between the trigger/safety pins and the fitting sub-assembly fasteners can damage the ends of the pins and thereby increase the risk of component failure. Orientation of latch fitting sub-assembly bolts with the threads inward and use of longer-than-required bolts exacerbates any interference between the assembly bolts and the trigger/safety pins. Failure of adjustment star retention springs increases the risk that the engine fan cowl latch tension will move out of adjustment. By design, the height of the latches and their position at the bottom centreline of the fan cowls makes it difficult to visually detect an unlatched latch. The present maintenance requirements regarding latch tensioning do not appear to be adequate to ensure that latch tension remains in accordance with the manufacturer's specified minima. The effectiveness of the flight crew's pre-flight inspection was lost when the inspection was done before completion of the maintenance activity. On 15 August 2000, the TSB forwarded Aviation Safety Advisory A000026-1 to Transport Canada suggesting that Transport Canada ensure that all Canadian-registered aircraft fitted with fan cowl latch fitting sub-assemblies have the fastener bolts correctly installed; advise Canadian air carriers of the increased safety that is attained by making latch handles more conspicuous; advise or issue a warning to Canadian air carriers regarding the dangers of using tools to operate latches not designed for tool use; advise Canadian air carriers of the TSB's survey results regarding eye-bolt adjustment retention spring failures and take appropriate action to correct the deficiency; advise Canadian air carriers of the importance of ensuring proper latch tensioning and of its effect on the operation and the integrity of the latching system; advise Canadian air carriers of the potential for incomplete trigger/safety engagement on Airbus A320 aircraft and monitor accordingly; and advise applicable foreign airworthiness authorities of the concerns raised in this advisory. In response to the safety advisory, Transport Canada notified the Federal Aviation Administration (FAA) certification office and the French Direction Gnral de l'Aviation Civile (DGAC), the organization responsible for the type design of Airbus aircraft, of the concerns indicated in the advisory; issued Service Difficulty Alert AL-2000-06 on 11 October 2000, providing five recommendations that could be taken to prevent future occurrences; and published an article entitled Large Aircraft Cowl Fastener Problems in issue 4/2000 of its newsletter Aviation Safety Maintainer. The article informed industry of the number of incidents involving cowl fastener failures, with specific mention of Airbus aircraft and a similar incident that took place on 13 September 2000 (TSB Report No A00O0199). In response to this occurrence, Canada 3000 checked and corrected the latch tensions in accordance with the aircraft maintenance manual; introduced an additional maintenance schedule requirement to recheck the latch tensions at C check intervals and at each engine change; added reflective tape to the sides and the face of each fan cowl latch on the A330's to make them more conspicuous in the unlatched position; introduced an additional maintenance schedule requirement on the A330 to recheck the reflective tape at A checks; and checked the A330 fleet for potential interference between the hook latching pins and the adjacent latch mounting bolts and corrected as necessary. A330, A320, and Boeing 757 Fleet Types Quality Control published inspection alerts for all fleet types, highlighting the importance of inspecting the latches for condition and checking for sufficient tension when closing; Quality Control made fan cowl latches a required inspection item (requiring a second inspection and signature each time they are opened) on all aircraft types in the fleet; Quality Control circulated Service Difficulty Alert No AL-2000-06 to heighten maintenance personnel's awareness of fan cowl latches; and Quality Control made TSB Aviation Safety Advisory A000026-1 (Factors Affecting the Loss of Engine Fan Cowls) the subject of a presentation at its annual Maintenance Conference in September 2000, again to heighten awareness. In response to this occurrence, BF Goodrich issued a service bulletin (CF6-80E1-NAC-71-032) on 01 January 2001 covering the correction to the direction of latch fitting sub-assembly bolts; will add a warning to the applicable maintenance manuals regarding potential damage to the latching systems from unauthorized use of tools to operate the latches; and revised the 3000-flight-hour periodic interval to the C-Check interval, although the March 2000 maintenance manual includes tension verification after inlet and fan cowl removal/replacement and periodic tension verifications at the 3000-flight-hour internal. This aircraft maintenance manual improvement to the C-Check interval was scheduled for the January 2002 revision.Safety Action Taken On 15 August 2000, the TSB forwarded Aviation Safety Advisory A000026-1 to Transport Canada suggesting that Transport Canada ensure that all Canadian-registered aircraft fitted with fan cowl latch fitting sub-assemblies have the fastener bolts correctly installed; advise Canadian air carriers of the increased safety that is attained by making latch handles more conspicuous; advise or issue a warning to Canadian air carriers regarding the dangers of using tools to operate latches not designed for tool use; advise Canadian air carriers of the TSB's survey results regarding eye-bolt adjustment retention spring failures and take appropriate action to correct the deficiency; advise Canadian air carriers of the importance of ensuring proper latch tensioning and of its effect on the operation and the integrity of the latching system; advise Canadian air carriers of the potential for incomplete trigger/safety engagement on Airbus A320 aircraft and monitor accordingly; and advise applicable foreign airworthiness authorities of the concerns raised in this advisory. In response to the safety advisory, Transport Canada notified the Federal Aviation Administration (FAA) certification office and the French Direction Gnral de l'Aviation Civile (DGAC), the organization responsible for the type design of Airbus aircraft, of the concerns indicated in the advisory; issued Service Difficulty Alert AL-2000-06 on 11 October 2000, providing five recommendations that could be taken to prevent future occurrences; and published an article entitled Large Aircraft Cowl Fastener Problems in issue 4/2000 of its newsletter Aviation Safety Maintainer. The article informed industry of the number of incidents involving cowl fastener failures, with specific mention of Airbus aircraft and a similar incident that took place on 13 September 2000 (TSB Report No A00O0199). In response to this occurrence, Canada 3000 checked and corrected the latch tensions in accordance with the aircraft maintenance manual; introduced an additional maintenance schedule requirement to recheck the latch tensions at C check intervals and at each engine change; added reflective tape to the sides and the face of each fan cowl latch on the A330's to make them more conspicuous in the unlatched position; introduced an additional maintenance schedule requirement on the A330 to recheck the reflective tape at A checks; and checked the A330 fleet for potential interference between the hook latching pins and the adjacent latch mounting bolts and corrected as necessary. A330, A320, and Boeing 757 Fleet Types Quality Control published inspection alerts for all fleet types, highlighting the importance of inspecting the latches for condition and checking for sufficient tension when closing; Quality Control made fan cowl latches a required inspection item (requiring a second inspection and signature each time they are opened) on all aircraft types in the fleet; Quality Control circulated Service Difficulty Alert No AL-2000-06 to heighten maintenance personnel's awareness of fan cowl latches; and Quality Control made TSB Aviation Safety Advisory A000026-1 (Factors Affecting the Loss of Engine Fan Cowls) the subject of a presentation at its annual Maintenance Conference in September 2000, again to heighten awareness. In response to this occurrence, BF Goodrich issued a service bulletin (CF6-80E1-NAC-71-032) on 01 January 2001 covering the correction to the direction of latch fitting sub-assembly bolts; will add a warning to the applicable maintenance manuals regarding potential damage to the latching systems from unauthorized use of tools to operate the latches; and revised the 3000-flight-hour periodic interval to the C-Check interval, although the March 2000 maintenance manual includes tension verification after inlet and fan cowl removal/replacement and periodic tension verifications at the 3000-flight-hour internal. This aircraft maintenance manual improvement to the C-Check interval was scheduled for the January 2002 revision.